1. Technical Field
The present invention concerns a grease injection system for sealing around flexible wireline while performing wireline service operations in a well. The invention specifically concerns an injection system in which the grease is continuously injected at a constant pressure into a grease control head.
2. Description of the Related Art
When using a grease control head, grease or high viscosity fluid is pumped or injected into the control head to lubricate the line running through the control head and to form a pressure barrier in the very close clearance between tubes in the control head and wireline running through the tubes. The barrier prevents pressured fluids from escaping from the well around the wireline.
The quantity of grease injected must be sufficient to maintain the pressure barrier and replace any grease carried out of the control head by clinging to the line when running line through the control head. Pressure on the grease must be above pressure in the well before grease can be injected into the control head.
Reciprocating piston pumps are usually used to inject grease directly into control heads. These pumps are equipped with discharge pressure limit switches set at a range of about 1000-2000 psi above well pressure and must be capable of producing discharge pressures of over 15,000 psi if the well pressure is in the 13,000 to 14,000 psi range. When the piston pump is running, and the wireline is not running through the control head, the quantity of grease discharged from the pump and injected will be more than the quantity of grease needed when the line is being run through the control head. Part of the excess injected grease will flow downwardly inside the control head into the well and part of the grease flows upwardly and out of the control head. Grease flowing out of the control head is usually conducted into a waste grease reservoir for disposal.
When the discharge pressure controlled pump is not running and line is running through the control head, grease is being carried out on the line and not replaced. When enough grease is carried away, pressure in the well will blowout the insufficient grease barrier and leakage will occur around the line.
As the dichsarge pressure of a piston pump fluctuates from a low on the suction stroke to a high on the discharge stroke, well pressure may blowout the grease barrier from the control head if injection pressure falls below well pressure long enough when the piston pump is on the suction stroke, and well pressure will leak around the line.